The present invention generally relates to a plasma CVD system, and more particularly to a plasma CVD system suitable for forming thin films onto relatively large substrates by the plasma CVD technique.
Conventionally, a plasma CVD system has been utilized to form a thin film for a relatively small substrate such as a silicon substrate for semiconductor devices. However, it is a recent trend to use the plasma CVD system to form a thin film to a substrate of a large area, e.g., a large-scale liquid crystal panel or the like. Moreover, although most of the conventional plasma CVD system is of a type processing for every predetermined number of substrates at one time in a so-called batch processing, such a system has been developed lately that automatically handles the substrates, that is, sequentially sends the substrates to a processing chamber to continuously perform a film forming process, a heating process as a pretreatment, and a cooling process as a posttreatment.
In FIG. 6 is shown the structure of a plasma CVD system used heretofore in the manufacture of large scale liquid crystal panels. A plurality of substrates 1 each formed of glass or the like are held by a pair of right and left trays 2. The substrate trays 2 are mounted to a conveyor 2a driven via a motor and a gear mechanism. The substrates 1 while mounted to the trays 2 are sequentially and continuously sent to a processing chamber 3 and sent out from the chamber 3 after the treatment is completed. A sheathed heater panel 4 is provided in the gap between the right and left trays 2 in the processing chamber 3, which heats the substrates 1 from the rear side. At the same time, a gas exhaust plate 5 serving also as an electrode is provided at the right and left outside of the substrate tray 2. The gas exhaust plate 5 is connected to a high frequency power source. Reaction gas is fed into the rear space of the gas exhaust plate 5. When the reaction gas is jetted out to the substrate 1 from the front side of the gas exhaust plate 5 as a voltage is impressed between the gas exhaust plate 5 and the substrate tray 2, a thin film is formed on the surface of the substrate 1.
Referring to FIG. 7, in the case where a plurality of processing chambers 3 are arranged side by side, the conveyor 2a is arranged in the form of a loop to circulate through the plurality of processing chambers 3. As indicated at the lower left in FIG. 7, the substrate tray 2 is suspended from the conveyor 2a after the substrates 1 are set on the substrate tray 2. When the conveyor 2a is started, the substrate tray 2 is passed through each processing chamber 3, where a predetermined treatment is performed to the substrates 1.
The conventional plasma CVD system of the above-described type accompanies an unfavorable problem that a thin film is formed on the substrate tray, drops and adheres to the surface of the substrate, thereby deteriorating the quality of the substrate surface.
More specifically, in the above plasma CVD system, a thin film is formed both on the substrate tray 2 and on the substrate 1. Since the substrate tray 2 is repeatedly used while circulating and moving in each processing chamber 3 and the external space, the tray is subjected to heating, vacuum heating, film formation, and cooling for every processing cycle and accordingly affected by the severe change of the temperature and atmospheric pressure. As a result, the thin film adhered to the substrate tray 2 is delaminated, i.e., scaling is caused. If the thin film detached from the substrate tray 2 adheres to the surface of the substrate 1, the surface quality of the substrate 1 is worsened. Moreover, a thin film is formed also on the surface of the structural member such as a transfer device, e.g., conveyor 2a or the like, and is slipped off as well.
Although it is good to remove the thin film adhered to the substrate tray 2 or the transfer device so as to prevent the adhesion of the delaminated thin film to the surface of the substrate 1, the removal requires considerably much time and labor, thus lowering the operation rate of the plasma CVD system and increasing the maintenance cost.